1. Technical Field
The present invention relates to a power factor correction type switching power supply unit that converts an alternating current voltage into a direct current voltage, and in particular relates to a power factor correction type switching power supply unit having a transient response correcting function that swiftly raises the output voltage when the output voltage decreases to or below a certain threshold value.
2. Related Art
In recent years, a switching power supply unit that has an alternating current voltage as an input has been widely utilized in electronic instruments. This kind of switching power supply unit is one which, by causing a switching operation of a switching element linking an input and an output, converts a full-wave rectified alternating current input voltage into a direct current output voltage of a desired size, and supplies it to a load.
FIG. 12 is a circuit diagram showing one example of a heretofore known power factor correction type switching power supply unit. Herein, a power factor correction (PFC) type switching power supply circuit that operates in continuous conduction mode is shown, and this is applied to an active filter type power supply unit.
The heretofore known power factor correction type switching power supply unit shown in FIG. 12 has a full-wave rectifier 4 that full-wave rectifies a commercial power supply 2, and its output is connected to one end of an inductor L1. The connection point of the other end of the inductor L1 and a diode D1 is connected to the drain terminal of, for example, an N-channel type MOS transistor (a metal oxide semiconductor field-effect transistor) configuring a switching element 6. The other end of the inductor L1 is connected to a load 8 via a rectifying and smoothing circuit formed of the diode D1 and a capacitor C1, and a direct current output voltage Vout is output to the load 8.
As well as the source terminal of the MOS transistor, which is the switching element 6, being connected to the ground (GND), the gate terminal is connected to an output terminal DO of a power factor correction control circuit 10. One end of a series resistance circuit formed of resistors R1 and R2 is connected to the connection point of the full-wave rectifier 4 and inductor L1, and the other end is grounded. A multiplier input terminal VDET of the power factor correction control circuit 10 is a terminal into which a detected value of an alternating current input voltage full-wave rectified by the full-wave rectifier 4 is input, and the connection point of the resistors R1 and R2 is connected to the multiplier input terminal VDET. Also, the full-wave rectifier 4 is grounded via a resistor R3, and the connection point of the full-wave rectifier 4 and resistor R3 is connected to an inductor current signal generating input terminal IS of the power factor correction control circuit 10. Furthermore, a series circuit of resistors R4 and R5 is connected in parallel with the load 8, and a direct current output voltage Vout the same as that of the load 8 is applied thereto. A feedback voltage input terminal FB of the power factor correction control circuit 10 being a terminal into which a detected value of the direct current output voltage Vout is input, herein, the connection point of the resistors R4 and R5 is connected to the feedback voltage input terminal FB, and a voltage signal wherein the direct current output voltage Vout is voltage divided is returned here.
Next, a simple description will be given of an operation of the heretofore known power factor correction type switching power supply unit of FIG. 12.
The heretofore known power factor correction type switching power supply unit of FIG. 12 employs a control method called an average current control method, average current mode control, or the like, and the power factor correction control circuit 10 is one that controls a current flowing to the alternating current commercial power supply 2 side into a sinusoidal wave in the same phase as that of the alternating current input voltage, while stabilizing the direct current output voltage Vout. The feedback voltage input terminal FB of the power factor correction control circuit 10 is connected to an input terminal of an operational transconductance amplifier (OTA) 15 configuring a voltage error amplifier circuit 14, together with a reference voltage source 12 that sets a voltage command value for the direct current output voltage Vout. The voltage error amplifier circuit 14 is configured of the OTA 15, a capacitor C2 for converting a current from the OTA 15 into a voltage, and a series circuit of a resistor R6 and capacitor C3 for phase compensation, each connected between the output terminal of the OTA 15 and a GND. The voltage error amplifier circuit 14 generates a voltage error signal Ver wherein the difference between the detected value of the direct current output voltage Vout (in this case, a divided voltage value) Vfb and the voltage command value (for example, 2.5V) of the reference voltage source 12 is amplified. The voltage error signal Ver is supplied to a first input terminal of a multiplier 24.
A second input terminal of the multiplier 24 is connected to the multiplier input terminal VDET of the power factor correction control circuit 10, and a detected value (a divided voltage value in this case) Vdet of the alternating current input voltage full-wave rectified by the full-wave rectifier 4 is input from here. The voltage error signal Ver supplied to the first input terminal and the detected value Vdet of the alternating current input voltage supplied to the second input terminal are multiplied in the multiplier 24, becoming the value of a current command (a current reference signal) Vmul to a current error amplifier circuit 26.
The current error amplifier circuit 26 is also connected to an inductor current signal generating input terminal IS via an inversion amplifier circuit 27. A voltage signal, which is an inductor current IL voltage converted in the current detecting resistor R3, is input into the inductor current signal generating input terminal IS, and an inductor current signal inversion amplified in the inversion amplifier circuit 27 is input into the current error amplifier circuit 26, along with the current reference signal Vmul. A current error signal wherein the differential voltage of the current reference signal Vmul and inductor current signal is amplified is output from the current error amplifier circuit 26.
As the voltage signal from the inductor current signal generating input terminal IS is of a negative potential, the inversion amplifier circuit 27 is provided in order to convert the voltage signal into an inductor current signal with a positive potential. As the function and configuration of the inversion amplifier circuit 27 itself are commonly known, a description thereof will be omitted. Also, in an oscillator circuit (OSC) 28, a sawtooth wave or triangular wave of a constant frequency is generated as a carrier signal that determines a switching cycle, and input into a PWM comparator 30. The PWM comparator 30 into which the carrier signal and the current error signal are input generates a pulse width modulation (PWM) control signal by comparing the magnitudes of the signals, and this is applied to the gate terminal of the switching element 6 via an AND circuit 32 and a driver circuit 34.
Herein, an overcurrent protection (OCP) circuit 36 is connected to the output side of the inversion amplifier circuit 27. The overcurrent protection (OCP) circuit 36 limits the maximum value of the inductor current IL based on the inductor current signal, which is the output of the inversion amplifier circuit 27. Herein, when an inductor current IL exceeding a predetermined threshold value flows, an L (low) level overcurrent limit signal is input into the AND circuit 32, and the output of the AND circuit 32 compulsorily becomes L. As a switching signal is output to the output terminal DO of the power factor correction control circuit 10 from the AND circuit 32 via the driver circuit 34, the switching element 6 is turned off on the output of the AND circuit 32 becoming L. By controlling the on-off timing of the switching element 6 in this way, it is possible to control the value of a current flowing to the capacitor C1 via the diode D1. Actually, a feedback constant setting circuit is connected between the input and output terminals of the current error amplifier circuit 26, but a depiction of the feedback constant setting circuit is omitted from FIG. 12.
In order to improve the power factor, that is, in order to cause the waveform of the current flowing to the alternating current commercial power supply 2 side to conform exactly to a waveform of a cycle of between 50 and 60 Hz of the alternating current input voltage, it is necessary to have the response characteristics of the power factor correction type switching power supply unit at around 10 Hz. However, as the improvement of the power factor is in a trade-off relationship with transient response characteristics with respect to a load fluctuation, or the like, there is a problem in a power supply unit used in a kind of application that requires transient response characteristics.
As a method of correcting the transient response characteristics, it is possible to respond by instantaneously raising the output of the voltage error amplifier circuit 14 when the direct current output voltage Vout decreases (for example, refer to U.S. Patent Application Publication No. 2007/0253223 and FIGS. 19A and 19B, Paragraphs [0119] to [0122] thereof). In the power factor correction control circuit 10, in the same way as in the one in U.S. Patent Application Publication No. 2007/0253223, a transient response corrector circuit 38 configured of a reference voltage source 16, a comparator 18, a constant current source 20, and a P-channel type MOSFET 22 is used. In the transient response corrector circuit 38, a second reference voltage (2.4V) lower than the voltage command value (2.5V) of the reference voltage source 12 is output from the reference voltage source 16, and it is determined that the direct current output voltage Vout has decreased. Then, on it being determined that the direct current output voltage Vout has decreased, a constant current is injected from the constant current source 20 into the capacitor C2 connected to the output side of the OTA 15, and the voltage error signal Ver to the multiplier 24 is compulsorily increased. Because of this, the output of the voltage error amplifier circuit 14 is instantaneously raised, and the direct current output voltage Vout is increased. However, according to this method, as the voltage error signal Ver is increased while ignoring the response characteristics of the power factor correction type switching power supply unit, there is a problem in that the inductor current IL and the direct current output voltage Vout may rise excessively, and an abnormal voltage be applied to the load 8.
In response to this, an overvoltage protection (OVP) circuit has been used to date as a way of dealing with the fluctuation of the direct current output voltage Vout (for example, refer to JP-A-2009-165316 and Paragraphs [0026] to [0076], and FIGS. 1 to 5 therein). This circuit, when the output voltage becomes excessive, causes an overvoltage protection function to operate with respect to the switching element 6 in order that the direct current output voltage Vout does not overshoot, and completely stops the switching operation of the switching element 6 after a certain time, or instantaneously (the switching element 6 is turned off). However, when this is applied to the power factor correction type switching power supply unit together with the transient response corrector circuit 38, there is a repetition of switching stopped→output reduction→switching restart with OVP omitted→output increase→overshoot→switching operation stopped by OVP function.
Also, in the transient response corrector circuit 38, the transient response correcting function operates regardless of the size of the alternating current input voltage. In the event that the transient response corrector circuit 38 functions at a start-up time or when the alternating current input voltage is high, the increase rate of the inductor current IL proportional to the voltage error signal and input voltage becomes too high. In this kind of case, as the transient response characteristics are originally reduced, the inductor current IL overshoots, entering an overcurrent protection condition, and a squeaking occurs at a time of the overcurrent protection operation of the overcurrent protection circuit 36. Alternatively, when providing an input filter having an unshown inductor for the commercial power supply 2, an input voltage drop is caused in the event that too much instantaneous power is removed, and it may happen that damage is caused to each element, and a malfunction occurs.
When correcting the transient response characteristics in the power factor correction type switching power supply unit in this way, simply raising the response characteristics of the output voltage feedback loop when the direct current output voltage decreases conversely causes an adverse effect, and may result in a reduction of efficiency and power factor.